Since the `redo` action never goes back to `index: 0`,
we have to mark the clean index as being the current
non-empty index for the undo/redo navigation to work properly.
The problem is that if we never `undo`, the stack index stays at zero,
which is the empty container waiting for commands. In that situation,
if we save the document, it registers the clean index as being 1
(the non-empty index) but because the stack index has never left zero,
the document was being reported as modified, being out of sync with
the window modified state.
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